1. Field of the Invention
The present invention relates to an optical system for displaying an image by scanning a light flux from a light source by using a scanning device and a scan-type image display apparatus provided with the same.
2. Description of the Related Art
Some electronic viewfinder apparatuses to be used in head-mounted-type display apparatuses (hereinafter, referred to as head-mounted displays), digital cameras and the like use a two-dimensional display element, which is so called a flat panel display, such as a transmission-type liquid crystal element, reflection-type liquid crystal elements, or organic EL elements. And, these structured by combining a two-dimensional display element with an ocular optical system for observing a displayed image as a virtual image have been proposed in large numbers.
In recent years, in such image display apparatuses, since higher definition images have been demanded, the number of pixels formed on their flat panel displays has also been increasing. However, with the increase in the number of pixels, pixel defects increase, and the size of the pixels is relatively reduced in comparison with the flat panel size, thus manufacturing becomes more difficult.
In addition, a scan-type image display apparatus using a scanning device without using a two-dimensional image display element has been disclosed in U.S. Pat. No. 5,467,104, etc. U.S. Pat. No. 5,467,104 discloses a technique which scans a light flux of R, G, and B in a horizontal direction and in a vertical direction and forms an image directly on the retina of an observer through an optical system.
However, in the scan-type image display apparatus disclosed in the above-described U.S. Pat. No. 5,467,104, optical scanning is required to be performed at very high speed, and therefore, a very small device is used as a scanning device for scanning light such as a mirror. Accordingly, the scanned light flux becomes extremely thin, and the light flux at the position of the pupil of the observer also becomes very small in diameter.
Techniques disclosed in U.S. Pat. No. 5,701,132 and in U.S. Pat. No. 5,757,544 can be mentioned as a method for enlarging such an exit pupil having a small diameter. In U.S. Pat. No. 5,701,132, by disposing an enlarging system, such as a lens array or a diffusing plate, on an intermediate image surface having a curvature formed by a scanned light flux, the divergence angle of a light flux which has passed through the enlarging system is increased.
Likewise, in U.S. Pat. No. 5,757,544, an exit pupil is enlarged by allowing light which has been incident into an image display element, such as a liquid crystal element illuminated with a parallel light flux, to pass through a diffusing plate, although the apparatus disclosed in this patent is not a scan-type image display apparatus.
However, in the structures disclosed in these U.S. patents, since the exit pupil enlarging system of the intermediate image transmission type is used, an optical path becomes long, and the apparatus is enlarged in size.
On the other hand, with the advance of semiconductor processes, it becomes possible to realize, by a Micro Electro Mechanical System (MEMS), a technique to manufacture a small-sized, lightweight, and high-speed scanning device.
Scanning devices manufactured by use of semiconductor processes have been disclosed in Japanese Patent Application Laid-Open No. H07(1995)-175005 (corresponding to U.S. Pat. No. 5,606,447) and Japanese Patent Application Laid-Open No. H08(1996)-334723. Also, similarly, in SPIE, Conference #4407-19 (June 2001), “Wafer scale packaging for a MEMS video scanner”, there is an introduction concerning a MEMS of a two-dimensional scanning device for a head-mounted-type display apparatus.
In the techniques disclosed in U.S. Pat. No. 5,701,132 and in U.S. Pat. No. 5,757,544, since the exit pupil enlarging system of the intermediate image transmission type is used, an optical path becomes long, however, for avoidance thereof, folding the optical path by using a light diffusing member of a reflection type can be considered. Owing to this effect, an optical system can be structured in a small size, however, on the other hand, since an optical path of light which is incident on the reflection-type light diffusing member and an optical path of light which is reflected and diffused are overlapped, a separating means for separating the optical paths is required. Although using a semi-transmission-type mirror or the like can be considered as a means for separating the optical paths, loss in the light amount occurs in the semi-transmission-type mirror.
On the other hand, by making good use of diffusing characteristics of light, a structure as shown in FIG. 1 can be considered as a means for separating optical paths. A description will be given for a case where a light flux 505 from an unillustrated light source is scanned by means of a scanning device 502 on a screen 503. A structure has been provided so that, through an unillustrated optical system, the light flux 505 forms a light source image on a screen 503, and when the scanning device 502 tilts around an axis 501 as its center, this scans light as shown by 504a, 504b and 504c in the drawing. If diffusion is provided for the light reflective member (screen) 503, reflected light is diffused as shown by diffused light fluxes 506a and 506b and is reflected in directions different from those of the incident light fluxes 504a to 504c. If an optical path separating member 507 such as a mirror with a hole is placed in the surroundings of the scanning device 502, only a greatly diffused light component out of the diffused light fluxes 506a and 506b is reflected, therefore, it becomes possible to separate the incident light flux from the diffused emergent light flux with a small light amount loss. However, like the incident light flux 504a, a light flux which proceeds to the center of the screen 503 becomes a component, out of the diffused light fluxes 506a and 506b, close to a regular reflection, and since this component does not hit the optical path separating member 507, an amount equivalent thereto results in a light amount loss.
Accordingly, in order to reduce the loss caused by an optical path separation, it is important to reduce the area of an opening to install the scanning device 502 in the optical path separating member 507 by, for example, downsizing the scanning device 502 as much as possible.
However, in the MEMS scanning devices disclosed in Japanese Patent Application Laid-Open No. H07 (1995)-175005 and Japanese Patent Application Laid-Open No. H08 (1996)-334723, etc., a torsion bar exists around the scanning surface (reflective surface), and if a mirror with a hole is used as a light separating member, this hole (opening portion) increases in size.
Moreover, particularly in the MEMS scanning devices described in Japanese Patent Application Laid-Open No. H08 (1996)-334723 and SPIE, Conference #4407-19 (June 2001), “Wafer scale packaging for a MEMS video scanner”, etc., a cover glass exists at a position distant from the scanning surface in some cases, wherein the necessity for increasing the opening of the light separating member in size arises, thus it is difficult to make the light separating member function effectively.